Ka-band Waveguide Ferrite Switches and Switch Matrices
The defence market has particular requirements for highly reliable, fast switching, high power handling switches and switch matrices in Ka-band. This is the sector targeted by Thales MESL when extending its portfolio of microwave switch products by developing Ka-band waveguide ferrite switches and switch matrices for short range air/sea defence systems. These switches use toroidal phase shifter technology to provide high power handling and fast switching as well as incorporating additional features, such as power splitting and selectable output polarization.
Typical applications are in fire control radar systems, as these radars, linked into command centres, can have multiple antennas for identification, tracking and ordinance uplinks. Therefore, using a switching network allows the radar designer to use one power source for multiple applications within the radar. A second common application at this frequency is for secure communications. In this application the switching networks can be used with the reverse function to that used for fire control radars — rather than one power source being switched between multiple antennas, several power sources can be switched to one output.
In order to get an insight into the development of the new products first consider that ferrite switches fill the gap between PIN diode switches (low power handling, fast switching speed) and rotary switches (high power handling, slow switching speed). Thales MESL produces two types of latching ferrite switches in WR28 waveguide: the switching junction circulator and the toroidal ferrite switch. The switching junction circulator is similar to the classic waveguide circulator that uses a ferrite resonator between permanent magnets.
In the switching circulator, the ferrite is magnetised by a current pulse fed through a wire threading the resonator, allowing the direction of circulation to be reversed. Without active cooling, this type of switch is limited to tens of watts of average power due to heat dissipation within the small ferrite resonator. Figure 1 shows a matrix that is constructed using five switching circulators and three high power loads. The driver electronics required to control the matrix are located on the rear side of the mounting plate.
The toroidal switch, in its simplest form, uses dual 90° toroidal ferrite phase shifters between a waveguide tee and coupler in a Butler matrix configuration, shown schematically in Figure 2. The tee splits the input power into two signals of equal magnitude and phase. These pass through the phase shifters which introduce a relative phase shift of +90° or –90°, before being recombined in the coupler to either one of the output ports.
The magnetization of the ferrite elements is controlled through wires lacing the toroids. The ferrite elements are only partially magnetized, increasing the complexity of the driver circuit required to control the switch, but allowing compensation for changes in the properties of the ferrite over temperature, which is necessary to maintain good performance over a wide temperature range.
Careful choice of ferrite material is necessary to ensure optimum performance for the requirements of each particular application. Peak power handling is limited by the critical power level of the ferrite material, above which insertion loss increases due to excitation of spin-wave instabilities. Peak power handling can be extended at the expense of slightly increased insertion loss by the inclusion of rare earth element doping in the ferrite material.
Average power handling is governed by the thermal properties of the assembly and can be increased by modifying the phase shifter geometry to decrease the phase and insertion loss per unit length. The toroid is made longer to maintain the available differential phase, but the power is dissipated over a larger area, limiting the heating of the ferrite material.
The second key aspect of switch design is the driver electronics. The magnetisation of the ferrite toroids is controlled by applying short (1 to 2 μs) current pulses through wires lacing the toroids. No holding current is required. It is necessary to compensate for changes in the magnetic properties of the ferrite material with temperature by adjusting the lengths of the current pulses according to a signal from a temperature sensor mounted in the metalwork close to one of the toroids. The switch driver interface is typically the power supplies (±15 V, 100 mA), one or more control lines and a status line from the driver, which reports any faults. An example specification for a Ka-band switch is given in Table 1.
The switch is non-reciprocal — it provides reverse isolation of 20 dB minimum. Reciprocal versions of the switch are also realizable using a different microwave circuit.
Switch Matrix Products
Several of the SP2T switches described above have been combined to produce two types of switch matrices that provide switching from one input to one of four outputs. The versatility of this switch element means that it can be used to provide additional features. Figure 3 shows an SP4T switch that can also provide an equal amplitude, equal phase power split between the four output ports. Figure 4 shows an SP4T switch with square waveguide outputs that also provides selectable polarization output from one port via an orthomode transducer. Other network topologies can be accommodated to suit particular system requirements.
These new products have been specifically developed to meet the defence market’s requirements for switches and switch matrices in Ka-band that enable fast and reliable, high power switching between antennas or power sources. They can easily be configured into a network to provide switching between one input and multiple outputs, making them suitable for land, air and naval defence systems, particularly for fire control radar and secure communications applications.
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